This disclosure relates generally to the field of geomagnetic surveying. More particularly, the disclosure relates to method for predicting a local geomagnetic disturbance field so that geomagnetic surveys may be more accurately referenced to the true geomagnetic field.
Accurate knowledge of the local geomagnetic field is required for numerous applications, such as navigation, attitude determination and control of moving objects, pointing of antennas, directional drilling, magnetic surveying, location of buried objects, magnetic signature reduction, and magnetic anomaly detection, among others. For example, well placement by measurement while drilling (MWD) often uses the direction of Earth's geomagnetic field as a reference direction. To compute the geodetic azimuth of a bottom hole assembly (BHA), which is an assembly of various types of drilling tools, the MWD tool makes measurements of the Earth's magnetic field and the user relates the measurements to the geomagnetic reference field at the well drilling site. This requires accurate knowledge of the local geomagnetic reference field direction (with respect to a geodetic reference) and strength.
For applications in well placement, i.e., directional drilling, a method known in the art as Interpolated In-Field Referencing (IIFR) is described in U.S. Pat. No. 6,021,577 issued to Shiells et al. The IIFR method provides an estimate of a magnetic disturbance field at the drill site. The IIFR method is a method in which disturbance field variations at a drill site are inferred from Earth magnetic field variations measured at remote sites. The IIFR method includes the following contributions to the geomagnetic field:                (i) a constant difference between the geomagnetic field elements between the drill site and each of the remote sites (equation: ER(t1)=ERvar(t1)+ERb1);        (ii) a phase shift of the a daily (24 hour period) variation, due to the difference in geographic longitude between the drill site and each of the remote sites (the first term on the right side in equation for ERvar(t1)); and        (iii) a weighted average of the short-term “high-pass filtered” variations at the remote sites (the second term on the right side in equation for ERvar(t1).        
Correction (i) simply expresses the fact that there is a difference between the geomagnetic field at the drill site and each of the remote sites. The foregoing correction does not include the effects of the disturbance field.
A possible limitation of corrections (ii) and (iii) is that they assume that the geomagnetic variations at the drill site are identical to a weighted average of the variations at the remote sites (except for a phase shift of the 24 hour daily variation to account for longitude differences between the drill site and each of the remote sites). This assumption is not usually correct in practice.
The actual magnetic disturbance field is the sum of source fields, caused by electric currents in the ionosphere and magnetosphere, and secondary induced fields, caused by electromagnetic induction in the Earth and oceans. By computing the magnetic variations at the drill site from a weighted average of the variations at the remote sites, the IIFR method makes two assumptions that are frequently incorrect:                (a) that the source fields are identical at the drill site and the remote sites. This may be incorrect because the source fields vary spatially, in particular at high latitudes; and        (b) that induced fields are identical at the drill site and the remote sites. This may be incorrect because of differences in the subsurface electrical conductivity between the drill site and the remote sites. Electrical conductivity depends on the local composition and water content of the subsurface. Particularly in situations where the drill site is offshore and the remote sites are on-shore, differences in conductivity may be many orders of magnitude, leading to large differences in geomagnetic variations and a failure of the IIFR method to perform correctly.        